Scientists are asking: What is the best way to fight climate change?

A new study says: Plant many trees; one trillion trees, maybe more.

Swiss scientists say there is enough room for that many trees. In their report in the journal Science, they say even with existing cities and farmland, there is enough space for new trees to cover nine million square kilometers. That is about the size of the United States.

Trees take carbon dioxide out of the air, and in return, put oxygen back into the air.

The study estimated that over a long period of time, the trees could take in almost 750 billion tons of carbon dioxide from the atmosphere. That is about as much carbon as humans have put into the atmosphere in the past 25 years.What is so bad about carbon dioxide? The gas traps heat, causing temperatures to rise.

Much of the gain from planting trees will come quickly because trees remove more carbon from the air when they are younger, the researchers said. The place that has the best possibility for removing the most carbon is the tropics— the area close to the equator.

This method would also have the lowest cost, says the study co-author Thomas Crowther.

"This is by far — by thousands of times — the cheapest climate change solution" and the most effective, he said. Crowther is a climate change ecologist at the Swiss Federal Institute of Technology in Zurich.

The six nations with the most room for new trees are Russia, the United States, Canada, Australia, Brazil and China.

Before his study, Crowther thought that there were other more effective ways to fight climate change besides cutting emissions. One example could be for people to change their diets by no longer eating meat. But, he said, tree planting is far more effective because trees remove so much carbon dioxide from the air.Thomas Lovejoy works as a biologist at George Mason University. He was not part of the study. But, he called the report "a good news story" because planting trees would also help stop the loss of biodiversity.

Crowther said planting trees is not a substitute for reducing the world's dependence on oil, coal and natural gas. The carbon dioxide produced by burning these fuels is believed to cause the warming of the atmosphere.

"None of this works without emissions cuts," he said.

And it is not easy or realistic to think the world will suddenly start planting a lot of trees quickly, although many groups have started, Crowther said.

"It's certainly a monumental challenge, which is exactly the scale of the problem of climate change," he said.

The researchers used Google Earth to see what areas could support more trees, while leaving room for people and crops. Lead writer for the study, Jean-Francois Bastin, estimated there is space for at least 1 trillion more trees, but it could be 1.5 trillion.

That is in addition to the 3 trillion trees that now live on Earth -- a number Crowther found in earlier research.

Source：VOA Special English